Accumulating evidence suggests that RNA-binding proteins play important roles in cell function and development. The goal of this application is to understand the function of the RNA-binding protein QKT in myelination. Diminished QKI expression in myelin-producing cells leads to severe dysmyelination in quakingviable (qkv) mice. QKI is a member of the Signal Transduction Activators of RNA (STARs), which carries a single RNA-binding domain as well as several Src-Homology 3 (SH3)-binding domains thus can interact with both RNA and signaling molecules. STAR proteins, upon phosphorylation in response to signaling cascades, are postulated to exert regulatory influences on cellular RNAs. Consistent with this view, we have found that QKI selectively interacts with the mRNA encoding the myelin basic protein (MBP), and tyrosine phosphorylation of QK1 dramatically reduces this interaction. The functional importance of this interaction is reinforced by our recent finding that MBP mRNA is severely destabilized and mislocalized in the qkv oligodendrocytes in which QKI is almost completely lost. These findings suggest that MBP mRNA is a functional target for QKI in myelination, and the interaction between QKI and the MBP mRNA is critical in controlling the normal posttranscriptional fate of the MBP mRNA. This application focuses on delineating the molecular mechanisms by which QKI regulates the metabolism of the MBP mRNA. Three specific aims are proposed: 1) To determine whether accelerated degradation of MBP mRNA occurs in the cytoplasm of qkt about/qkv oligodendrocytes, and whether elevated QKI expression prolongs the half-life of the MBP mRNA; 2) To define the MBP mRNA element required for interaction with QKI and to determine whether this element mediates QIU' s effect on mRNA stability; 3) To test whether tyrosine-phosphorylation of QKI regulates its ability to bind and to stabilize mRNA. Answers to these questions should significantly advance our knowledge of fundamental mechanisms governing myelination and provide particular insights into how mRNA metabolism is controlled by protein-RNA interaction. This may ultimately lead to new therapeutic strategies against myelin disorders. In addition, understanding how QKI regulates its RNA targets during myelination may elucidate common mechanisms for other STARs in cell growth and tumorigenesis.